In a color video camera, white balance is achieved when a neutral white object imaged by the camera under given illumination is represented as red (R), green (G), and blue (B) signals having equal output levels. White balance is needed since the RGB representation produced by a color video camera typically changes as the illumination of a scene varies. In some circumstances, a color video camera white balanced for certain illumination conditions will not be white balanced for other illumination conditions. As a result, it is possible that an object under two different illuminations will have two different RGB representations even though a human observer would perceive the object as having the same color under both illuminations.
In a manual operation, white balance is achieved by imaging a neutral white object under the illumination of interest and adjusting the amplification of one or more of the red, green, and blue signals until their respective output levels are equal. In an automatic white balance (AWB) operation, a neutral white object under the illumination of interest is imaged and the amplification levels of each of the red and blue signals are adjusted. For example, the output levels of the red and blue signals may be made equal to that of the green signal. In both operations, maintenance of white balance will depend upon the consistency of the illumination conditions and maintenance of the adjusted amplification levels.
In an automatic tracing white balance (ATW) operation, the white balance operation is automatically repeatedly carried out during an ordinary imaging process. Since an ordinary imaged scene may not contain a neutral white object, it is possible that the white balance will be incorrectly adjusted with reference to a colored object. Consequently, true white balance may not be achieved, e.g. a non-white color is represented as the color white.
FIG. 9 illustrates a black body radiation curve BBR, plotted on red signal gain vs. blue signal gain axes relative to red and blue signal amplifiers in a video camera. As shown, signal gain values are represented by eight-bits and, accordingly, each axis extends from 0 to 256. The red signal gain and blue signal gain axes intersect at the point (128, 128). Also illustrated is a white area, indicated by slanted lining, which is generally symmetric about the black body radiation curve BBR. The white area represents red and blue signal values which are characteristic of the color white.
To avoid the problem of incorrect white balance adjustment during an ATW operation, the red and blue signal values representing an imaged scene may be compared to a predetermined set of red and blue signal values characteristic of the color white. The white area comprises such a set of red and blue signal values characteristic of the color white.
Previously, it was attempted to manually calibrate the white area such that output levels of the respective R, G, and B signals are mutually equal when a reference light source is imaged. With reference to FIG. 9, calibration with respect to a standard light source would be attained when the B signal gain and the R signal gain are both equal to 128.
In practice, a variable resistance was provided to manually adjust white balance amplification while a standard light source was imaged. A manual adjustment was made of the R signal and B signal output levels to make them equal to the G signal output level. Since the adjustment was made manually by a user or technician, errors in calibration have occurred and a precise adjustment has been difficult to achieve. Miscalibration introduces error into the ATW operation, reducing its effectiveness. Additionally, the introduction of manual adjustment error may significantly limit further improvement in ATW processing efforts. Further error is introduced if the setting of the variable resistance shifts after a manual adjustment. Repeated manual readjustment can be difficult.